Pneumatic brake booster, in particular for a motor vehicle, comprising a reaction disk (30) interposed between a plunger (28) carried by a control rod actuated by the brake pedal and a thrust rod (32) acting on the primary piston of a tandem master cylinder, the reaction disk (30) being housed inside an annular metal part (36) which cooperates with a stop (50) of the thrust rod (32) by the intermediary of thermal expansion element (48) designed to compensate for the thermal expansion of the reaction disk (30) and its effect on a point of change of assistance ratio.